Gold plating has been applied to the surface of industrial electronics parts or articles, such as print wiring board, ceramic IC package, ITO base board, IC card, etc., due to favorable properties of gold, such as electric conductivity, soldering capacity, physical property (e.g., connection by thermal pressure), resistance to oxidation and chemical stability. Many of these parts or articles are required to be gold plated at an electrically independent area. Therefore, electric gold plating is not suitable and non-electrolytic gold plating method has to be used.
Currently two methods are available: a method using a substitution gold plating liquid by which gold deposits as the base metal, such as nickel, dissolves; and autocatalytic type gold plating by which gold is produced from gold compounds by reducing agents wherein gold itself has a catalytic action for it. These two types of non-electrolytic gold plating liquids are widely known.
In the case of the substitution gold plating, gold deposits by substituting the base metal, namely, the base metal dissolves (etching or corrosion) as gold deposits. Currently available substitution gold plating liquids are unable to control the rate of substitution reaction, as a result, the substitution rate is very high at the onset of reaction. Especially, many defect spots on the substituted gold layer are produced right after the reaction due to that fast substitution reaction, causing continuous defect spots or localized defect area. Etching or corrosion on the base metal under the defect gold plating layer progresses vertically deep or horizontally wide excessively. Consequently, in the case where the gold plating is carried out using such a substitution gold plating liquid, a part of the base metal where there is structurally weak crystalline grain boundary is dissolved (etching and corrosion) preferentially and convergently. Accordingly, it is known that in the case where gold plating is carried out by use of the currently available substitution gold plating liquid, deep crevasse-like etching along the grain boundary or wide horizontal corrosion develops excessively in the base metal after the gold plating layer is formed.
For example, when general non-electrolytic nickel or gold plating is carried out using the known non-electrolytic nickel plating bath or substitution gold plating bath, scanning electron microscopic examination of a slice of substitution gold layer of 0.05 to 0.1 .mu.m thickness on non-electrolytically plated nickel layer of 0.5 .mu.m revealed that the gold plating liquid preferentially attacked the deposited grain boundary portion of the non-electrolytically formed nickel layer, causing deep corrosion at the grain boundary, resulting in the formation of a cavity under the gold layer. Although the thickness of the gold layer is only 0.1 .mu.m or less, the depth of corrosion is 3 to 5 .mu.m. Such weakening of the non-electrolytically plated nickel layer after the substitution gold plating and unsatisfactory adherence between the gold layer and nickel layer makes the resultant product unendurable to soldering and hence impractical.
Also, the autocatalytic type gold plating is a two-step process: right after immersion of the base metal to be plated in the plating liquid, gold deposits by substitution reaction between the base metal and gold ion, and then the deposited gold initiates gold-catalyzed reducing action, causing sedimentation of gold. Accordingly, in the case of the autocatalytic type gold plating, it is not possible to prevent etching and corrosion of the base metal caused by gold plating liquid.
Such a plated layer with insufficient adherence is prone to peel off during efficacy tests or is unable to provide strength for soldering, resulting in exposure of the base metal after soldering and during soldering strength tests. Recently, however, a ball grid array type semiconductor package manufactured by using print board wiring technique is widely used as a package for microprocessor. In a ball grid array type semiconductor package, it is necessary to perform gold plating on an electrically independent pattern to improve soldering strength. However, there is a big problem on production of defect products due to inadequate soldering strength in the currently available non-electrolytic gold plating technology. Therefore, electric plating technology is still being used to attain necessary soldering strength.